Analyte  sensors with a sensing surface having small sensing spots

ABSTRACT

Embodiments of the present disclosure relate to analyte determining methods and devices (e.g., electrochemical analyte monitoring systems) that have a sensing surface that includes an array two or more discontiguous sensing elements deposited on a substrate surface, where the sensing elements comprise one or more droplets of a sensing element formulation.

INTRODUCTION

In many instances it is desirable or necessary to regularly monitor theconcentration of particular constituents in a fluid. A number of systemsare available that analyze the constituents of bodily fluids such asblood, urine and saliva. Examples of such systems conveniently monitorthe level of particular medically significant fluid constituents, suchas, for example, cholesterol, ketones, vitamins, proteins, and variousmetabolites or blood sugars, such as glucose. Diagnosis and managementof patients suffering from diabetes mellitus, a disorder of the pancreaswhere insufficient production of insulin prevents normal regulation ofblood sugar levels, requires carefully monitoring of blood glucoselevels on a daily basis. A number of systems that allow individuals toeasily monitor their blood glucose are currently available. Such systemsinclude electrochemical biosensors, including those that comprise aglucose sensor that is adapted for insertion into a subcutaneous sitewithin the body for the continuous monitoring of glucose levels inbodily fluid of the subcutaneous site (see for example, U.S. Pat. No.6,175,752 to Say et al).

A person may obtain a blood sample by withdrawing blood from a bloodsource in his or her body, such as a vein, using a needle and syringe,for example, or by lancing a portion of his or her skin, using a lancingdevice, for example, to make blood available external to the skin, toobtain the necessary sample volume for in vitro testing. The person maythen apply the fresh blood sample to a test strip, whereupon suitabledetection methods, such as calorimetric, electrochemical, or photometricdetection methods, for example, may be used to determine the person'sactual blood glucose level. The foregoing procedure provides a bloodglucose concentration for a particular or discrete point in time, andthus, must be repeated periodically, in order to monitor blood glucoseover a longer period.

In addition to the discrete or periodic, in vitro, bloodglucose-monitoring systems described above, at least partiallyimplantable, or in vivo, blood glucose-monitoring systems, which areconstructed to provide continuous in vivo measurement of an individual'sblood glucose concentration, have been described and developed.

Such analyte monitoring devices are constructed to provide forcontinuous or automatic monitoring of analytes, such as glucose, in theblood stream or interstitial fluid. Such devices include electrochemicalsensors, at least a portion of which are operably positioned in a bloodvessel or in the subcutaneous tissue of a user.

While continuous glucose monitoring is desirable, there are severalchallenges associated with optimizing manufacture protocols to improveyield and uniformity of the sensing elements of the biosensorsconstructed for in vivo use. Accordingly, further development ofmanufacturing techniques and methods, as well as analyte-monitoringdevices, systems, or kits employing the same, is desirable.

SUMMARY

Embodiments of the present disclosure relate to analyte determiningmethods and devices (e.g., electrochemical analyte monitoring systems)that have a sensing surface that includes two or more sensing elementsdisposed laterally to each other, where the sensing surface is on aworking electrode of in vivo and/or in vitro analyte sensors, e.g.,continuous and/or automatic in vivo monitoring using analyte sensorsand/or test strips. Also provided are systems and methods of using the,for example electrochemical, analyte sensors in analyte monitoring.

Aspects of the present disclosure include an analyte sensor thatincludes: a working electrode; and a counter electrode, where theworking electrode includes a sensing surface having two or more sensingelements disposed laterally to each other, where each sensing elementincludes an analyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, at least a portion of the analyte sensor isadapted to be subcutaneously positioned in a subject.

In some instances, the analyte sensor further includes a membranedisposed over the sensing elements that limits flux of analyte to thesensing elements.

In some cases, the analyte-responsive enzyme includes aglucose-responsive enzyme. In certain instances, the sensing elementsinclude a redox mediator. For example, the redox mediator may include aruthenium-containing complex or an osmium-containing complex.

In certain embodiments, the analyte sensor is a glucose sensor. In somecases, the analyte sensor is an in vivo sensor. In other cases, theanalyte sensor is an in vitro sensor.

Aspects of the present disclosure also include a method for monitoring alevel of an analyte in a subject. The method includes positioning atleast a portion of an analyte sensor into skin of a subject, anddetermining a level of an analyte over a period of time from signalsgenerated by the analyte sensor, where the determining over a period oftime provides for monitoring the level of the analyte in the subject. Asdescribed above, the analyte sensor includes: a working electrode; and acounter electrode, wherein the working electrode includes a sensingsurface having two or more sensing elements disposed laterally to eachother, where each sensing element includes an analyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, at least a portion of the analyte sensor isadapted to be subcutaneously positioned in a subject.

In some instances, the analyte sensor further includes a membranedisposed over the sensing elements that limits flux of analyte to thesensing elements.

In some cases, the analyte-responsive enzyme includes aglucose-responsive enzyme. In certain instances, the sensing elementsinclude a redox mediator. For example, the redox mediator may include aruthenium-containing complex or an osmium-containing complex.

In certain embodiments, the analyte sensor is a glucose sensor. In somecases, the analyte sensor is an in vivo sensor. In other cases, theanalyte sensor is an in vitro sensor.

Aspects of the present disclosure further include a method formonitoring a level of an analyte using an analyte monitoring system. Themethod includes: inserting at least a portion of an analyte sensor intoskin of a patient; attaching an analyte sensor control unit to the skinof the patient; coupling a plurality of conductive contacts of theanalyte sensor control unit to a plurality of contact pads of theanalyte sensor; collecting data, using the analyte sensor control unit,regarding a level of an analyte from signals generated by the analytesensor; and transmitting the collected data from the analyte sensorcontrol unit to a receiver unit. As described above, the analyte sensorincludes a working electrode and a counter electrode, where the workingelectrode includes a sensing surface having two or more sensing elementsdisposed laterally to each other, where each sensing element includes ananalyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, the analyte is glucose.

In some instances, the collecting data includes generating signals fromthe analyte sensor and processing the signals into data. In certaincases, the data comprise the signals from the analyte sensor.

In certain embodiments, the method further includes activating an alarmif the data indicate an alarm condition. In some cases, the methodfurther includes administering a drug in response to the data. Forexample, the drug may be insulin.

In certain instances, the method does not include a calibration step.

Aspects of the present disclosure also include a method of fabricatingan electrode for use in an analyte sensor. The method includescontacting a sensing surface of a working electrode with two or moresensing elements disposed laterally to each other, where each sensingelement comprises an analyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, the method is a method of fabricating two ormore electrodes for use in a plurality of analyte sensors. In theseembodiments, the method includes contacting a sensing surface on each ofthe two or more electrodes with two or more sensing elements disposedlaterally to each other, wherein each sensing element comprises ananalyte-responsive enzyme. In some instances, the electrodes have acoefficient of variation in sensitivity of 8% or less.

In certain cases, at least a portion of the analyte sensor is adapted tobe subcutaneously positioned in a subject. In some instances, theanalyte sensor further includes a membrane disposed over the sensingelements.

In certain embodiments, the analyte-responsive enzyme includes aglucose-responsive enzyme. In some cases, the sensing elements include aredox mediator. For example, the redox mediator may include aruthenium-containing complex or an osmium-containing complex.

In certain instances, the analyte sensor is a glucose sensor. In somecases, the analyte sensor is an in vivo sensor. In other instances, theanalyte sensor is an in vitro sensor.

In certain embodiments of the method, the contacting includes depositingone or more drops comprising the sensing elements onto the sensingsurface of the working electrode. In some cases, the method furtherincludes contacting the sensing elements with a membrane that limitsflux of analyte to the sensing elements.

Aspects of the present disclosure also include an analyte test stripthat includes: a first substrate having a first surface; a secondsubstrate having a second surface opposing the first surface, the firstand second substrates being disposed so that the first surface is infacing relationship with the second surface; a working electrodedisposed on the first surface; and a counter electrode disposed on oneof the first surface and the second surface, where the working electrodeincludes a sensing surface having two or more sensing elements disposedlaterally to each other, where each sensing element includes ananalyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, the analyte test strip further includes a spacerbetween the first substrate and the second substrate.

In some cases, the analyte-responsive enzyme includes aglucose-responsive enzyme. In certain instances, the sensing elementsincludes a redox mediator. For example, the redox mediator may include aruthenium-containing complex or an osmium-containing complex.

In certain embodiments, the analyte test strip is a glucose test strip.

Aspects of the present disclosure further include a method formonitoring a level of an analyte in a subject. The method includescontacting a sample from a subject to an analyte test strip anddetermining a level of an analyte from a signal generated by the analytetest strip, where the determining provides for monitoring the level ofthe analyte in the subject. As described above, the analyte test stripincludes: a first substrate having a first surface; a second substratehaving a second surface opposing the first surface, the first and secondsubstrates being disposed so that the first surface is in facingrelationship with the second surface; a working electrode disposed onthe first surface; and a counter electrode disposed on one of the firstsurface and the second surface, where the working electrode includes asensing surface having two or more sensing elements disposed laterallyto each other, where each sensing element includes an analyte-responsiveenzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, the method further includes a spacer between thefirst substrate and the second substrate.

In some cases, the analyte-responsive enzyme includes aglucose-responsive enzyme. In certain instances, the sensing elementsinclude a redox mediator. For example, the redox mediator may include aruthenium-containing complex or an osmium-containing complex.

In certain embodiments, the analyte test strip is a glucose test strip.

Aspects of the present disclosure also include a method for monitoring alevel of an analyte using an analyte monitoring system. The methodincludes: coupling a conductive contact of an analyte test strip to ananalyte monitoring system; contacting a sample from a subject to theanalyte test strip; collecting data, using the analyte monitoringsystem, regarding a level of an analyte from a signal generated by theanalyte test strip; and determining a level of an analyte from thecollected data, where the determining provides for monitoring the levelof the analyte in the subject. As discussed above, the analyte teststrip includes: a first substrate having a first surface; a secondsubstrate having a second surface opposing the first surface, the firstand second substrates being disposed so that the first surface is infacing relationship with the second surface; a working electrodedisposed on the first surface; and a counter electrode disposed on oneof the first surface and the second surface, where the working electrodeincludes a sensing surface having two or more sensing elements disposedlaterally to each other, where each sensing element includes ananalyte-responsive enzyme.

In certain embodiments, the sensing elements are discontiguous. In somecases, the sensing elements are arranged as individual sensing elementson the working electrode. For example, the sensing surface may includean array of two or more individual sensing elements. In some cases, thesensing surface includes an array of 100 or more individual sensingelements. In certain instances, the sensing surface has a density ofsensing elements ranging from 2-1000 sensing elements/mm².

In certain embodiments, the sensing surface further includesinter-feature areas. The inter-feature areas may surround the sensingelements. In some instances, the sensing elements have an inter-featuredistance ranging from 1 μm to 500 μm. In certain cases, theinter-feature areas are free of the analyte-responsive enzyme. In somecases, the inter-feature areas are free of a redox mediator.

In certain embodiments, the working electrode further includes a secondlayer of two or more sensing elements disposed on the sensing surface.In some instances, the sensing elements have an average diameter of 200μm or less.

In certain embodiments, the analyte is glucose.

In some instances of the method, the collecting data includes generatingthe signal from the analyte test strip and processing the signal intodata. In certain cases, the data include the signals from the analytetest strip.

In certain instances, the method further includes activating an alarm ifthe data indicate an alarm condition. In some cases, the method furtherincludes administering a drug in response to the data. For example, thedrug may be insulin.

In certain embodiments, the method does not include a calibration step.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of various embodiments of the present disclosureis provided herein with reference to the accompanying drawings, whichare briefly described below. The drawings are illustrative and are notnecessarily drawn to scale. The drawings illustrate various embodimentsof the present disclosure and may illustrate one or more embodiment(s)or example(s) of the present disclosure in whole or in part. A referencenumeral, letter, and/or symbol that is used in one drawing to refer to aparticular element may be used in another drawing to refer to a likeelement.

FIG. 1 shows a block diagram of an embodiment of an analyte monitoringsystem according to embodiments of the present disclosure.

FIG. 2 shows a block diagram of an embodiment of a data processing unitof the analyte monitoring system shown in FIG. 1.

FIG. 3 shows a block diagram of an embodiment of the primary receiverunit of the analyte monitoring system of FIG. 1.

FIG. 4 shows a schematic diagram of an embodiment of an analyte sensoraccording to the embodiments of the present disclosure.

FIGS. 5A-5B show a perspective view and a cross sectional view,respectively, of an embodiment an analyte sensor. FIG. 5C shows aschematic view of a working electrode according to embodiments of thepresent disclosure.

FIG. 6 shows a photograph of a working electrode coated with six sensingelements with a radius of approximately 150 μm each at a distance ofapproximately 150 μm from each other. The resulting sensors have acoefficient of variation in sensitivity of 5% or less.

FIG. 7A shows a perspective view of an embodiment of an analyte sensorthat has an array of sensing elements in substantially aligned rowsaccording to embodiments of the present disclosure. FIG. 7B shows aperspective view of another embodiment of an analyte sensor that has anarray of sensing elements in offset rows according to embodiments of thepresent disclosure.

FIG. 8 shows a perspective view of an embodiment of an analyte sensorthat has an array of sensing elements in offset rows with minimalinter-feature areas according to embodiments of the present disclosure.

FIG. 9 shows a perspective view of an embodiment of an analyte sensorthat has layered arrays of sensing elements according to embodiments ofthe present disclosure.

FIG. 10 shows a cross-sectional view of a working electrode that has aplurality of sensing elements on the surface of a working electrodeaccording to embodiments of the present disclosure.

FIG. 11 shows an exploded perspective view of an analyte sensor teststrip, the layers illustrated individually with the electrodes in afirst configuration according to embodiments of the present disclosure.

FIG. 12 shows an exploded perspective view of an analyte sensor teststrip, the layers illustrated individually with the electrodes in asecond configuration according to embodiments of the present disclosure.

FIG. 13 shows a graph of current (μA) vs. time (seconds) for a sensinglayer formulation deposited as an array of small sensing elements vs. astripe coating according to embodiments of the present disclosure.

DETAILED DESCRIPTION

Before the embodiments of the present disclosure are described, it is tobe understood that this invention is not limited to particularembodiments described, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the embodiments of the invention will beembodied by the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassedwithin the invention. The upper and lower limits of these smaller rangesmay independently be included or excluded in the range, and each rangewhere either, neither or both limits are included in the smaller rangesis also encompassed within the invention, subject to any specificallyexcluded limit in the stated range. Where the stated range includes oneor both of the limits, ranges excluding either or both of those includedlimits are also included in the invention.

In the description of the invention herein, it will be understood that aword appearing in the singular encompasses its plural counterpart, and aword appearing in the plural encompasses its singular counterpart,unless implicitly or explicitly understood or stated otherwise. Merelyby way of example, reference to “an” or “the” “analyte” encompasses asingle analyte, as well as a combination and/or mixture of two or moredifferent analytes, reference to “a” or “the” “concentration value”encompasses a single concentration value, as well as two or moreconcentration values, and the like, unless implicitly or explicitlyunderstood or stated otherwise. Further, it will be understood that forany given component described herein, any of the possible candidates oralternatives listed for that component, may generally be usedindividually or in combination with one another, unless implicitly orexplicitly understood or stated otherwise. Additionally, it will beunderstood that any list of such candidates or alternatives, is merelyillustrative, not limiting, unless implicitly or explicitly understoodor stated otherwise.

Various terms are described below to facilitate an understanding of theinvention. It will be understood that a corresponding description ofthese various terms applies to corresponding linguistic or grammaticalvariations or forms of these various terms. It will also be understoodthat the invention is not limited to the terminology used herein, or thedescriptions thereof, for the description of particular embodiments.Merely by way of example, the invention is not limited to particularanalytes, bodily or tissue fluids, blood or capillary blood, or sensorconstructs or usages, unless implicitly or explicitly understood orstated otherwise, as such may vary.

The publications discussed herein are provided solely for theirdisclosure prior to the filing date of the application. Nothing hereinis to be construed as an admission that the embodiments of the inventionare not entitled to antedate such publication by virtue of priorinvention. Further, the dates of publication provided may be differentfrom the actual publication dates which may need to be independentlyconfirmed.

Systems and Methods Having Two or More Sensing Elements

Embodiments of the present disclosure relate to methods and devices forreducing variation in sensor sensitivity by including a sensing surfacethat includes two or more sensing elements disposed laterally to eachother, where the sensing surface is on a working electrode of thesensor, such as in vivo and/or in vitro analyte sensors, includingcontinuous and/or automatic in vivo analyte sensors. For example,embodiments of the present disclosure provide for a sensing surface of aworking electrode that includes an array of two or more individualsensing elements, resulting in a decrease in variation of the sensorsensitivity between individual sensors. Also provided are systems andmethods of using the analyte sensors in analyte monitoring.

Embodiments of the present disclosure are based on the discovery thatdeposition of two or more sensing elements disposed laterally to eachother on a sensing surface of a working electrode in the manufacture ofin vivo and/or in vitro biosensors reduces variation in sensorsensitivity between sensors. In certain embodiments, a sensor thatincludes two or more sensing elements disposed laterally to each otheron a sensing surface of a working electrode has a variation in sensorsensitivity that is lower than a sensor with one single larger sensingelement. Stated another way, sensors may have a lower variation insensor sensitivity for a sensor that includes two or more sensingelements disposed laterally to each other on a sensing surface of aworking electrode such that the total sensing element area per sensor isless than a sensor that has a single larger sensing element with agreater total sensing element area per sensor. In certain embodiments,sensors that include two or more sensing elements disposed laterally toeach other on a sensing surface of a working electrode have acoefficient of variation in sensitivity of 20% or less, such as 15% orless, including 10% or less, such as 8% or less, or 5% or less, or 3% orless, or 2% or less, or 1% or less.

During the manufacturing process for the subject analyte sensors, anaqueous solution (e.g., a sensing element formulation) is contacted witha surface of a substrate (e.g., a surface of a working electrode),forming a deposition of the solution (e.g., a sensing element) on thesurface of the substrate. The sensing elements may include ananalyte-responsive enzyme. In certain instances, the sensing elementsinclude a redox mediator. In some cases, the sensing element formulationis deposited such that the sensing elements are discontiguous. By“discontiguous” is meant that a sensing element does not share an edgeor boundary (e.g., is not touching) an adjacent sensing element. Forexample, the sensing elements may be arranged as individual (e.g.,discreet) sensing elements on the surface of the working electrode. Inother embodiments, the sensing elements are deposited on the surface ofthe working electrode such that the edges of the sensing elementscontact the edges of one or more adjacent sensing elements. In theseembodiments, the sensing elements may be referred to as “contiguous”.

In certain embodiments, the sensing surface includes an array of two ormore individual sensing elements on the working electrode. As usedherein, the term “array” refers to any one-dimensional, two-dimensionalor substantially two-dimensional (as well as a three-dimensional)arrangement of regions bearing a particular composition associated withthat region. In some instances, the arrays are arrays of a formulation,such as a sensing element formulation. As such, in some embodiments, thearrays are arrays of individual sensing elements, where each sensingelement includes a sensing element formulation.

Any given substrate may carry one, two, four or more arrays of sensingelements disposed on a surface of the substrate. Depending upon the use,any or all of the arrays may be the same or different from one anotherand each may contain multiple spots or features (e.g., sensingelements). For example, an array may include two or more, 5 or more, tenor more, 25 or more, 50 or more, 100 or more features, or even 1000 ormore features, in an area of 100 mm² or less, such as 75 mm² or less, or50 mm² or less, for instance 25 mm² or less, or 10 mm² or less, or 5 mm²or less, such as 2 mm² or less, or 1 mm² or less, 0.5 mm² or less, or0.1 mm² or less. For example, features may have widths (that is,diameter, for a round spot) in the range from 0.1 μm to 1 mm, or from 1μm to 1 mm, such as ranging from 1 μm to 500 μm, including from 10 μm to250 μm, for example from 50 μm to 200 μm. In certain embodiments, thesensing elements have an average diameter of 500 μm or less, such as 250μm or less, including 200 μm or less, or 150 μm or less, or 100 μm orless, such as 50 μm or less, or 10 μm or less, or 1 μm or less, or 0.1μm or less. Non-round features may have area ranges equivalent to thatof circular features with the foregoing width (diameter) ranges.

In certain embodiments, the sensing surface includes inter-featureareas. Inter-feature areas do not carry any sensing element formulation.As such, in some instances, the inter-feature areas do not include(e.g., are substantially free of) an analyte-responsive enzyme. Inaddition, in some cases, the inter-feature areas do not include (e.g.,are substantially free of) a redox mediator or polymer bound, covalentlyor non-covalently, redox mediator. The inter-feature areas maysubstantially surround the sensing elements, such that, as describedherein, the sensing elements are discontiguous. In some cases, thesensing elements have inter-feature areas, wherein the distance betweenadjacent sensing elements (e.g., the inter-feature distance) is suchthat the flux of analyte to a sensing element does not significantlyinterfere with the flux of analyte to adjacent sensing elements. Forexample, the inter-feature distance may be 0.1 μm or more, 0.5 μm ormore, 1 μm or more, such as 10 μm or more, including 50 μm or more, or100 μm or more, or 150 μm or more, or 200 μm or more, or 250 μm or more,for instance 500 μm or more. The inter-feature distance may range from0.1 μm to 500 μm, or from 0.5 μm to 500 μm, or from 1 μm to 500 μm, suchas from 1 μm to 250 μm, including from 5 μm to 200 μm, for instance from10 μm to 200 μm. Such inter-feature areas may be present where thearrays are formed by processes involving drop deposition of the sensingelement formulation onto a sensing surface of a working electrode, asdescribed in more detail below. It will be appreciated that theinter-feature areas, when present, could be of various sizes andconfigurations.

Each array may cover an area of 100 mm² or less, or 50 mm², or 25 mm² orless, such as 10 mm², 5 mm² or less, 1 mm² or less, or 0.1 mm² or less,or 0.01 mm² or less, for instance 0.001 mm² or less. In someembodiments, the sensing surface has a density of sensing elements of 2sensing elements/mm² or more, such as 5 sensing elements/mm² or more,including 10 sensing elements/mm² or more, or 50 sensing elements/mm² ormore, or 100 sensing elements/mm² or more, such as 250 sensingelements/mm² or more, including 500 sensing elements/mm² or more, or1000 sensing elements/mm² or more. For example, the sensing surface mayhave a density of sensing elements ranging from 2-1000 sensingelements/mm², such as 2-500 sensing elements/mm², including 2-250sensing elements/mm², or 2-100 sensing elements/mm², or 2-50 sensingelements/mm², such as 2-10 sensing elements/mm².

Arrays can be fabricated using drop deposition of the sensing elementformulation onto a sensing surface of a working electrode. For example,the sensing element formulation may be deposited by any non-impact orimpact printing method, such as, for example, from a pulse-jet device. A“pulse-jet” is a device that can dispense drops in the formation of anarray. Pulse jet devices operate by delivering a pulse of pressure toliquid adjacent an outlet or orifice such that a drop will be dispensedtherefrom (for example, by a piezoelectric or thermoelectric elementpositioned in the same chamber as the orifice). In certain embodiments,the drops may be dispensed using a dispenser device configured tooperate similar to an inkjet printing device, as described above. Incertain embodiments, the pulse-jet device includes a dispensing headconfigured to dispense drops, such as, but not limited to, sensing layerformulation, in the formation of an array. The dispensing head may be ofa type commonly used in an inkjet type of printer and may, for example,include one or more deposition chambers for containing theformulation(s) to be deposited. The amount of fluid that is deposited ina single activation event of a pulse jet can be controlled by changingone or more of a number of parameters, including the size of the orificein the dispensing head (e.g., the orifice diameter), the size of thedeposition chamber, the size of the piezoelectric or thermoelectricelement, etc. The amount of fluid deposited during a single activationevent may range from 0.01 to 1000 picoliters (pL), such as from 0.1 to750 pL, including from 1 to 500 pL, or form 1 to 250 pL, or from 1 to100 pL, for instance from 1 to 75 pL, or from 1 to 50 pL, such as from 1to 25 pL, or from 1 to 10 pL, for example from 1 to 5 pL. In certaincases, the amount of fluid deposited during a single activation eventmay range from 1 to 50 pL.

In certain embodiments, during the manufacturing process for the subjectanalyte sensors, an aqueous solution (e.g., a sensing layer formulation)is contacted with a surface of a substrate (e.g., a surface of a workingelectrode), forming a deposition of the solution on the surface of thesubstrate. In some cases, the solution is allowed to dry and cure.Without being limited to any particular theory, in certain instances,during the drying, the constituents of the solution may tend to migratetowards the outer edges of the deposition due to a faster rate ofevaporation at the thinner peripheral edges of the deposition. Thisresults in a greater concentration of the constituents of the solutionat the peripheral edges of the deposition, resulting in a so-called“coffee ring” effect. Analyte sensors are typically manufactured bydepositing a stripe or relatively large drop of a sensing layerformulation onto the surface of an electrode, which, in some cases, mayresult in a “coffee ring” effect as described above. For example, asdescribed above, when an elongated stripe of sensing layer formulationdries on the surface of the electrode, constituents in the sensing layerformulation may migrate towards the outer edges of the stripe, resultingin an uneven coating of the sensing layer formulation on the surface ofthe electrode with a higher concentration of the sensing layerformulation near the edges of the sensing layer stripe.

In certain embodiments of the present disclosure, the deposition of anarray of small sensing elements may result in a reduction, and in somecases, complete elimination of the “coffee ring” effect. For instance,the coffee-ring effect may be minimized by depositing an array of two ormore individual sensing elements on the working electrode. In somecases, due to their small size, the small sensing elements in the arrayhave a rate of evaporation that is greater than the rate of evaporationof a sensing layer formulation deposited as a stripe or a relativelylarge drop on the surface of the electrode. In certain embodiments, thefaster rate of evaporation results in a more uniform distribution of theconstituents of the solution deposited on the substrate upon drying andcuring as compared to a solution deposited as a relatively larger stripeor drop of the sensing layer formulation. This, in turn, may improve thecoefficient of variation and the overall manufacturing process of thesensor and overall system. In certain embodiments, small sensingelements may facilitate faster sensor fabrication due to faster dryingof the very small sensing element spots, even at room temperature.Drying time may further be decreased by drying the sensing elementsabove room temperature, such as at a temperature of 25-100° C., such as30-80° C., including 40-60° C.

In some instances, each sensing element (e.g., feature on the array) hasa volume ranging from 0.01 to 1000 picoliters (pL), such as from 0.1 to750 pL, including from 1 to 500 pL, or form 1 to 250 pL, or from 1 to100 pL, for instance from 1 to 75 pL, or from 1 to 50 pL, such as from 1to 25 pL, or from 1 to 10 pL, for example from 1 to 5 pL. In certaincases, each sensing element has a volume ranging from 1 to 50 pL. Asdescribed above, the array of sensing elements may be deposited on thesurface of the electrode such that there is an inter-feature areabetween each individual sensing element on the array, such that thesensing elements are discontiguous.

In certain embodiments, a single layer of sensing elements is depositedon the surface of the working electrode. In other cases, two or morelayers of sensing elements are deposited on the surface of the workingelectrode. For example, the working electrode may include a sensingsurface that includes a first layer of sensing elements as describedabove, and may further include a second layer of sensing elementsdisposed on the sensing surface. In these cases, the first layer may bedeposited as a first array of sensing elements on the surface of theworking electrode. A second layer of sensing elements may be depositedas a second array of sensing elements disposed on the first array ofsensing elements. In some cases, the second array of sensing elements isdeposited such that each sensing element in the second array issubstantially aligned on top of a corresponding sensing element of thefirst array of sensing elements. In other instances, the second array ofsensing elements is deposited such that each sensing element in thesecond array is deposited substantially on top of an inter-feature areaof the first array of sensing elements. In these instances, the secondarray of sensing elements may be offset from the positions of thesensing elements in the first array of sensing elements. In someinstances, the second layer of sensing elements may overlap at least aportion of one or more sensing elements in the underlying first layer ofsensing elements. The deposition of a first array and second array ofsensing elements in an offset configuration as described above mayfacilitate the formation of a contiguous coating of the sensing layerformulation on the surface of the working electrode. Additional layersof sensing elements may be deposited on the working electrode, eithersubstantially aligned with the underlying layer or offset from theunderlying layer, as desired. The deposition of multiple layers ofsensing elements on the surface of the working electrode may facilitatethe cumulative deposition of a desired total quantity of the sensinglayer formulation on the surface of the working electrode.

Without being limited to any particular theory, in certain instances,the sensitivity of an analyte sensor depends on the area of the sensinglayer, e.g., a layer disposed on a surface of a working electrode thatincludes a sensing formulation having an analyte-responsive enzyme, andin some cases a redox mediator or a redox mediator bound, covalently ornon-covalently, to a polymer. For sensing layers that are a contiguouslayer of the sensing layer formulation, the sensor sensitivity dependson the area of the sensing layer and does not significantly depend onedge effects of the sensing layer. For instance, the sensitivity of thesensor may depend on the flux of analyte through a flux limitingmembrane disposed over the sensing layer in a 2-dimensional mannertowards a surface of the working electrode (e.g., towards a planarsurface). In certain embodiments, inclusion of two or more sensingelements disposed laterally to each other allows the area of the sensingelements to be minimized, such that edge effects of the sensing layerare maximized. This may result in the sensor sensitivity being dependenton edge effects of the sensing elements, rather than the overall area ofthe sensing elements. As such, the sensitivity of the sensor may dependon the flux of analyte through a flux limiting membrane disposed overthe sensing elements in a radial 3-dimensional manner towards theworking electrode (e.g., towards a point). In certain cases, the sensingelements have an arcuate profile to promote radial diffusion of theanalyte through the flux limiting membrane disposed over the sensingelements towards the working electrode. For example, FIG. 10 shows across-sectional view of a working electrode 1000 that has a plurality ofsensing elements 1020 on the surface of a working electrode 1010. Thesensing elements 1020 have an arcuate cross-sectional profile configuredto promote radial diffusion (as shown by the arrows) of the analytethrough the flux limiting membrane 1030 disposed over the sensingelements 1020 towards the working electrode 1010.

In some instances, the sensing elements have an arcuate profile. By“arcuate” is meant that the cross-sectional profile of the sensingelements have an arc or rounded shape. In certain cases, the sensingelements have a shape approximating that of a half sphere, where therounded semi-spherical portion of the sensing element is convex andextends a distance above the surface of the substrate (e.g., the surfaceof the working electrode). In some instances, semi-spherical sensingelements may have a surface area that is greater that the surface areaof a typical substantially flat or non-semi-spherically shaped sensingelement. For example, semi-spherical sensing elements may have a surfacearea that is 1.1 or more times greater than the surface area of atypical substantially flat (e.g., non-semi-spherically shaped) sensingelement, such as 1.2 or more, including 1.3 or more, or 1.4 or more, or1.5 or more, or 1.6 or more, or 1.7 or more, or 1.8 or more, or 1.9 ormore, or 2 or more times greater than the surface area of a typicalsubstantially flat (e.g., non-semi-spherically shaped) sensing element.In certain embodiments, sensing elements that have a greater surfacearea may facilitate an increase in the surface area of the sensing layerformulation that is able to contact the analyte as the analyte diffusesthrough the flux limiting membrane towards the sensing elements.

In some instances, because the sensor sensitivity depends on edgeeffects, rather than the overall area of the sensing elements, smallrelative changes in the area of the sensing elements will notsignificantly affect the sensitivity of the sensor. In certainembodiments, this results in a decrease in variation of the sensorsensitivity. A decrease in variation of the sensor sensitivity mayfacilitate calibration of the sensor during the manufacturing process.For example, embodiments of sensors of the present disclosure may becalibrated during the manufacturing process, such that calibration ofthe sensors by a user is not required. As such, in some cases, systemsusing sensors of the present disclosure do not need to perform acalibration step prior to use of the sensors by the user for analytedetection.

An embodiment of a sensing element may be described as the area shownschematically in FIG. 5B as 508. The sensing element may be described asthe active chemical area of the biosensor. The sensing elementformulation, which can include a glucose-transducing agent, may include,for example, among other constituents, a redox mediator, such as, forexample, a hydrogen peroxide or a transition metal complex, such as aruthenium-containing complex or an osmium-containing complex, and ananalyte-responsive enzyme, such as, for example, a glucose-responsiveenzyme (e.g., glucose oxidase, glucose dehydrogenase, etc.) orlactate-responsive enzyme (e.g., lactate oxidase). The sensing elementmay also include other optional components, such as, for example, apolymer and a bi-functional, short-chain, epoxide cross-linker, such aspolyethylene glycol (PEG). As described herein, two or more sensingelements may be provided on a sensing surface of the working electrode,where the two or more sensing elements are disposed laterally to eachother. For example, FIG. 5C shows a schematic view of a portion ofworking electrode 501. Working electrode 501 includes a plurality ofindividual sensing elements 508. The sensing elements 508 arediscontiguous, such that the sensing elements 508 are arranged into anarray of individual sensing elements 508 on the working electrode 501.

FIG. 7A shows a schematic view of a portion of an analyte sensor 700that includes an array of sensing elements 710 deposited on a portion ofa working electrode 720. The array of sensing elements 710 is arrangedsuch that each row of sensing elements in the array is substantiallyaligned with the sensing elements in an adjacent row. As shown in FIG.7A, the sensing elements 710 are arranged into an array of individualdiscontiguous sensing elements on the working electrode 720. FIG. 7Bshows a schematic view of another embodiment of an analyte sensor 750.The portion of the analyte sensor 750 shown includes an array of sensingelement 760 deposited on a portion of a working electrode 770. The arrayof sensing elements 760 is arranged such that each row of sensingelements in the array is offset from the sensing elements in an adjacentrow. As shown in FIG. 7B, the sensing elements 760 are arranged into anarray of individual discontiguous sensing elements on the workingelectrode 770. In some instances, arranging the rows of sensing elementsin an offset configuration may facilitate the fabrication of an arraywith a greater density of sensing elements per unit area as compared toan array with rows of sensing elements substantially aligned, whilestill maintaining an array of individual discontiguous sensing elements.

As described above, in other embodiments, the array of sensing elementsmay be configured such that the inter-feature areas are minimized. Forexample, FIG. 8 shows an embodiment of an analyte sensor 800 thatincludes an array of sensing elements 810 disposed on a portion of aworking electrode 820. The array of sensing elements 810 is arrangedsuch that each row of sensing elements in the array is offset from thesensing elements in an adjacent row. As shown in FIG. 8, the sensingelements 810 are arranged such that the edges of the sensing elementsare in contact with one or more adjacent sensing elements. In someinstances, arranging the rows of sensing elements in an offsetconfiguration with the sensing elements in contact with one or moreadjacent sensing elements may facilitate the fabrication of an arraywith a greater density of sensing elements per unit area as compared toan array with discontiguous sensing elements.

As described above, in certain embodiments, two or more layers ofsensing elements may be deposited on the surface of a working electrode.For example, FIG. 9 shows a schematic of an analyte sensor 900 thatincludes sensing elements 910 and 930. Sensing elements 910 of a firstlayer are deposited as a first array on the surface of a workingelectrode 920. Sensing elements 930 of a second layer are deposited as asecond array disposed on the sensing elements 910 of the first array. Asshown in FIG. 9, the sensing elements 930 of the second array aredeposited such that each sensing element 930 in the second array isdeposited substantially on top of an inter-feature area of the sensingelements 910 of the first array. The sensing elements 930 of the secondarray are offset from the positions of the sensing elements 910 in thefirst array. The sensing elements 930 of the second array overlap atleast a portion of one or more sensing elements 910 in the underlyingfirst array (see expanded inset in FIG. 9). The deposition of sensingelements 910 in a first array and sensing elements 930 in a second arrayin an offset configuration as described above may facilitate theformation of a contiguous coating of the sensing layer formulation onthe surface of the working electrode 920. Additional layers of sensingelements may be deposited on the working electrode, either substantiallyaligned with the underlying layer or offset from the underlying layer,as desired.

In an electrochemical embodiment, the sensor is placed,transcutaneously, for example, into a subcutaneous site such thatsubcutaneous fluid of the site comes into contact with the sensor. Inother in vivo embodiments, placement of at least a portion of the sensormay be in a blood vessel. The sensor operates to electrolyze an analyteof interest in the subcutaneous fluid such that a current is generatedbetween the working electrode and the counter electrode. A value for thecurrent associated with the working electrode is determined. If multipleworking electrodes are used, current values from each of the workingelectrodes may be determined. A microprocessor may be used to collectthese periodically determined current values or to further process thesevalues.

If an analyte concentration is successfully determined, it may bedisplayed, stored, transmitted, and/or otherwise processed to provideuseful information. By way of example, raw signal or analyteconcentrations may be used as a basis for determining a rate of changein analyte concentration, which should not change at a rate greater thana predetermined threshold amount. If the rate of change of analyteconcentration exceeds the predefined threshold, an indication maybedisplayed or otherwise transmitted to indicate this fact.

As demonstrated herein, the methods of the present disclosure are usefulin connection with a device that is used to measure or monitor a glucoseanalyte, such as any such device described herein. These methods mayalso be used in connection with a device that is used to measure ormonitor another analyte (e.g., ketones, ketone bodies, HbA1c, and thelike), including oxygen, carbon dioxide, proteins, drugs, or anothermoiety of interest, for example, or any combination thereof, found inbodily fluid, including subcutaneous fluid, dermal fluid (sweat, tears,and the like), interstitial fluid, or other bodily fluid of interest,for example, or any combination thereof. In general, the device is ingood contact, such as thorough and substantially continuous contact,with the bodily fluid.

According to embodiments of the present disclosure, the measurementsensor is one suited for electrochemical measurement of analyteconcentration, for example glucose concentration, in a bodily fluid. Inthese embodiments, the measurement sensor includes at least a workingelectrode and a counter electrode. Other embodiments may further includea reference electrode. The working electrode may be associated with aglucose-responsive enzyme. A mediator may also be included. In certainembodiments, hydrogen peroxide, which may be characterized as amediator, is produced by a reaction of the sensor and may be used toinfer the concentration of glucose. In some embodiments, a mediator isadded to the sensor by a manufacturer, e.g., is included with the sensorprior to use. The redox mediator may be disposed relative to the workingelectrode and is capable of transferring electrons between a compoundand a working electrode, either directly or indirectly. The redoxmediator may be, for example, immobilized on the working electrode,e.g., entrapped on a surface or chemically bound to a surface.

Additional embodiments of a sensor that may include a working electrodewith a sensing surface that includes two or more sensing elementsdisposed laterally to each other are described in U.S. Pat. Nos.5,262,035, 5,262,305, 6,134,461, 6,143,164, 6,175,752, 6,338,790,6,579,690, 6,605,200, 6,605,201, 6,654,625, 6,736,957, 6,746,582,6,932,894, 7,090,756 as well as those described in U.S. patentapplication Ser. Nos. 11/701,138, 11/948,915, 12/625,185, 12/625,208,and 12/624,767, the disclosures of all of which are incorporated hereinby reference in their entirety. Moreover, the embodiments disclosedherein may be incorporated into battery-powered or self-powered analytesensors, such as self-powered analyte sensors, as disclosed in U.S.patent application Ser. No. 12/393,921 (U.S. Patent ApplicationPublication No. 2010/0213057), the disclosure of which is incorporatedby reference herein in its entirety. In addition, the embodimentsdisclosed herein may be incorporated into analyte monitoring systems anddevices that utilize one or more rivets to attach an analyte sensorhaving one or more conductive traces to a sensor control unit, such asdisclosed in U.S. Provisional Patent Application No. 61/498,142, filedJun. 17, 2011, the disclosure of which is incorporated by referenceherein in its entirety.

Aspects of the present disclosure also include embodiments that includea sensing surface that has two or more sensing elements disposedlaterally to each other, where the sensing surface is on a workingelectrode of an analyte test strip sensor. For example, FIG. 11 shows anexploded perspective view of an analyte sensor test strip, the layersillustrated individually with the electrodes in a first configuration.As shown in FIG. 11, test strip 1100 has a first substrate 1110, asecond substrate 1120, and a spacer 1130 positioned therebetween. Teststrip 1100 includes at least one working electrode 1140 and at least onecounter electrode 1160. The working electrode 1140 is present on asurface of the first substrate 1110 and the counter electrode 1160 ispresent on a surface of the second substrate 1120 opposing the surfaceof the first substrate 1110 in a facing relationship with the surface ofthe first substrate. The working electrode 1140 has an array of sensingelements 1150 disposed on the sensing surface of the working electrode1140. Test strip 1100 is a layered construction, in certain embodimentshaving a generally rectangular shape, e.g., its length is longer thanits width, although other shapes are possible as well. Anotherembodiment of a test strip is illustrated in FIG. 12, which shows anexploded perspective view of an analyte sensor test strip, the layersillustrated individually with the electrodes in a second configuration.As shown in FIG. 12, test strip 1200 has a first substrate 1210, asecond substrate 1220, and a spacer 1230 positioned therebetween. Teststrip 1200 includes at least one working electrode 1240 and at least onecounter electrode 1260. The counter electrode 1260 is present on asurface of the first substrate 1210 adjacent the working electrode 1240,such that both the working electrode 1240 and the counter electrode 1260are present on the surface of the first substrate 1210. The workingelectrode 1240 has an array of sensing elements 1250 disposed on thesensing surface of the working electrode 1240. Similar to the embodimentshown in FIG. 11, the test strip 1200 shown in FIG. 12 has a layeredconstruction, in certain embodiments having a generally rectangularshape, e.g., its length is longer than its width, although other shapesare possible as well. Additional embodiments of test strips and analytesensors for use therein are described in more detail in U.S. applicationSer. No. 11/281,883, the disclosure of which is incorporated byreference herein in its entirety.

Analyte test strips for use with the present devices can be of any kind,size, or shape known to those skilled in the art; for example,FREESTYLE® and FREESTYLE LITE™ test strips, as well as PRECISION™ teststrips sold by ABBOTT DIABETES CARE Inc. In addition to the embodimentsspecifically disclosed herein, the devices of the present disclosure canbe configured to work with a wide variety of analyte test strips, e.g.,those disclosed in U.S. patent application Ser. No. 11/461,725, filedAug. 1, 2006; U.S. Patent Application Publication No. 2007/0095661; U.S.Patent Application Publication No. 2006/0091006; U.S. Patent ApplicationPublication No. 2006/0025662; U.S. Patent Application Publication No.2008/0267823; U.S. Patent Application Publication No. 2007/0108048; U.S.Patent Application Publication No. 2008/0102441; U.S. Patent ApplicationPublication No. 2008/0066305; U.S. Patent Application Publication No.2007/0199818; U.S. Patent Application Publication No. 2008/0148873; U.S.Patent Application Publication No. 2007/0068807; U.S. patent applicationSer. No. 12/102,374, filed Apr. 14, 2008, and U.S. Patent ApplicationPublication No. 2009/0095625; U.S. Pat. Nos. 6,616,819; 6,143,164;6,592,745; 6,071,391 and 6,893,545; the disclosures of each of which areincorporated by reference herein in their entirety.

Electrochemical Sensors

Embodiments of the present disclosure relate to methods and devices fordetecting at least one analyte, including glucose, in body fluid.Embodiments relate to the continuous and/or automatic in vivo monitoringof the level of one or more analytes using a continuous analytemonitoring system that includes an analyte sensor at least a portion ofwhich is to be positioned beneath a skin surface of a user for a periodof time and/or the discrete monitoring of one or more analytes using anin vitro blood glucose (“BG”) meter and an analyte test strip.Embodiments include combined or combinable devices, systems and methodsand/or transferring data between an in vivo continuous system and an invivo system. In some embodiments, the systems, or at least a portion ofthe systems, are integrated into a single unit.

A sensor as described herein may be an in vivo sensor or an in vitrosensor (i.e., a discrete monitoring test strip). Such a sensor can beformed on a substrate, e.g., a substantially planar substrate. Incertain embodiments, the sensor is a wire, e.g., a working electrodewire inner portion with one or more other electrodes associated (e.g.,on, including wrapped around) therewith. The sensor may also include atleast one counter electrode (or counter/reference electrode) and/or atleast one reference electrode or at least one reference/counterelectrode.

Accordingly, embodiments include analyte monitoring devices and systemsthat include an analyte sensor at least a portion of which ispositionable beneath the skin surface of the user for the in vivodetection of an analyte, including glucose, lactate, and the like, in abody fluid. Embodiments include wholly implantable analyte sensors andanalyte sensors in which only a portion of the sensor is positionedunder the skin and a portion of the sensor resides above the skin, e.g.,for contact to a sensor control unit (which may include a transmitter),a receiver/display unit, transceiver, processor, etc. The sensor may be,for example, subcutaneously positionable in a user for the continuous orperiodic monitoring of a level of an analyte in the user's interstitialfluid. For the purposes of this description, continuous monitoring andperiodic monitoring will be used interchangeably, unless notedotherwise. The sensor response may be correlated and/or converted toanalyte levels in blood or other fluids. In certain embodiments, ananalyte sensor may be positioned in contact with interstitial fluid todetect the level of glucose, which detected glucose may be used to inferthe glucose level in the user's bloodstream. Analyte sensors may beinsertable into a vein, artery, or other portion of the body containingfluid. Embodiments of the analyte sensors may be configured formonitoring the level of the analyte over a time period which may rangefrom seconds, minutes, hours, days, weeks, to months, or longer.

In certain embodiments, the analyte sensors, such as glucose sensors,are capable of in vivo detection of an analyte for one hour or more,e.g., a few hours or more, e.g., a few days or more, e.g., three or moredays, e.g., five days or more, e.g., seven days or more, e.g., severalweeks or more, or one month or more. Future analyte levels may bepredicted based on information obtained, e.g., the current analyte levelat time to, the rate of change of the analyte, etc. Predictive alarmsmay notify the user of a predicted analyte level that may be of concernin advance of the user's analyte level reaching the future predictedanalyte level. This provides the user an opportunity to take correctiveaction.

FIG. 1 shows a data monitoring and management system such as, forexample, an analyte (e.g., glucose) monitoring system 100 in accordancewith certain embodiments. Aspects of the subject disclosure are furtherdescribed primarily with respect to glucose monitoring devices andsystems, and methods of glucose detection, for convenience only and suchdescription is in no way intended to limit the scope of the embodiments.It is to be understood that the analyte monitoring system may beconfigured to monitor a variety of analytes at the same time or atdifferent times.

Analytes that may be monitored include, but are not limited to, acetylcholine, amylase, bilirubin, cholesterol, chorionic gonadotropin,glycosylated hemoglobin (HbA1c), creatine kinase (e.g., CK-MB),creatine, creatinine, DNA, fructosamine, glucose, glucose derivatives,glutamine, growth hormones, hormones, ketones, ketone bodies, lactate,peroxide, prostate-specific antigen, prothrombin, RNA, thyroidstimulating hormone, and troponin. The concentration of drugs, such as,for example, antibiotics (e.g., gentamicin, vancomycin, and the like),digitoxin, digoxin, drugs of abuse, theophylline, and warfarin, may alsobe monitored. In embodiments that monitor more than one analyte, theanalytes may be monitored at the same or different times.

The analyte monitoring system 100 includes an analyte sensor 101, a dataprocessing unit 102 connectable to the sensor 101, and a primaryreceiver unit 104. In some instances, the primary receiver unit 104 isconfigured to communicate with the data processing unit 102 via acommunication link 103. In certain embodiments, the primary receiverunit 104 may be further configured to transmit data to a data processingterminal 105 to evaluate or otherwise process or format data received bythe primary receiver unit 104. The data processing terminal 105 may beconfigured to receive data directly from the data processing unit 102via a communication link 107, which may optionally be configured forbi-directional communication. Further, the data processing unit 102 mayinclude a transmitter or a transceiver to transmit and/or receive datato and/or from the primary receiver unit 104 and/or the data processingterminal 105 and/or optionally a secondary receiver unit 106.

Also shown in FIG. 1 is an optional secondary receiver unit 106 which isoperatively coupled to the communication link 103 and configured toreceive data transmitted from the data processing unit 102. Thesecondary receiver unit 106 may be configured to communicate with theprimary receiver unit 104, as well as the data processing terminal 105.In certain embodiments, the secondary receiver unit 106 may beconfigured for bi-directional wireless communication with each of theprimary receiver unit 104 and the data processing terminal 105. Asdiscussed in further detail below, in some instances, the secondaryreceiver unit 106 may be a de-featured receiver as compared to theprimary receiver unit 104, for instance, the secondary receiver unit 106may include a limited or minimal number of functions and features ascompared with the primary receiver unit 104. As such, the secondaryreceiver unit 106 may include a smaller (in one or more, including all,dimensions), compact housing or embodied in a device including a wristwatch, arm band, PDA, mp3 player, cell phone, etc., for example.Alternatively, the secondary receiver unit 106 may be configured withthe same or substantially similar functions and features as the primaryreceiver unit 104. The secondary receiver unit 106 may include a dockingportion configured to mate with a docking cradle unit for placement by,e.g., the bedside for night time monitoring, and/or a bi-directionalcommunication device. A docking cradle may recharge a power supply.

Only one analyte sensor 101, data processing unit 102 and dataprocessing terminal 105 are shown in the embodiment of the analytemonitoring system 100 illustrated in FIG. 1. However, it will beappreciated by one of ordinary skill in the art that the analytemonitoring system 100 may include more than one sensor 101 and/or morethan one data processing unit 102, and/or more than one data processingterminal 105. Multiple sensors may be positioned in a user for analytemonitoring at the same or different times. In certain embodiments,analyte information obtained by a first sensor positioned in a user maybe employed as a comparison to analyte information obtained by a secondsensor. This may be useful to confirm or validate analyte informationobtained from one or both of the sensors. Such redundancy may be usefulif analyte information is contemplated in critical therapy-relateddecisions. In certain embodiments, a first sensor may be used tocalibrate a second sensor.

The analyte monitoring system 100 may be a continuous monitoring system,or semi-continuous, or a discrete monitoring system. In amulti-component environment, each component may be configured to beuniquely identified by one or more of the other components in the systemso that communication conflict may be readily resolved between thevarious components within the analyte monitoring system 100. Forexample, unique IDs, communication channels, and the like, may be used.

In certain embodiments, the sensor 101 is physically positioned in or onthe body of a user whose analyte level is being monitored. The sensor101 may be configured to at least periodically sample the analyte levelof the user and convert the sampled analyte level into a correspondingsignal for transmission by the data processing unit 102. The dataprocessing unit 102 is coupleable to the sensor 101 so that both devicesare positioned in or on the user's body, with at least a portion of theanalyte sensor 101 positioned transcutaneously. The data processing unitmay include a fixation element, such as an adhesive or the like, tosecure it to the user's body. A mount (not shown) attachable to the userand mateable with the data processing unit 102 may be used. For example,a mount may include an adhesive surface. The data processing unit 102performs data processing functions, where such functions may include,but are not limited to, filtering and encoding of data signals, each ofwhich corresponds to a sampled analyte level of the user, fortransmission to the primary receiver unit 104 via the communication link103. In some embodiments, the sensor 101 or the data processing unit 102or a combined sensor/data processing unit may be wholly implantableunder the skin surface of the user.

In certain embodiments, the primary receiver unit 104 may include ananalog interface section including an RF receiver and an antenna that isconfigured to communicate with the data processing unit 102 via thecommunication link 103, and a data processing section for processing thereceived data from the data processing unit 102 including data decoding,error detection and correction, data clock generation, data bitrecovery, etc., or any combination thereof.

In operation, the primary receiver unit 104 in certain embodiments isconfigured to synchronize with the data processing unit 102 to uniquelyidentify the data processing unit 102, based on, for example, anidentification information of the data processing unit 102, andthereafter, to periodically receive signals transmitted from the dataprocessing unit 102 associated with the monitored analyte levelsdetected by the sensor 101.

Referring again to FIG. 1, the data processing terminal 105 may includea personal computer, a portable computer including a laptop or ahandheld device (e.g., a personal digital assistant (PDA), a telephoneincluding a cellular phone (e.g., a multimedia and Internet-enabledmobile phone including an iPhone™, a Blackberry®, or similar phone), anmp3 player (e.g., an iPOD™, etc.), a pager, and the like), and/or a drugdelivery device (e.g., an infusion device), each of which may beconfigured for data communication with the receiver via a wired or awireless connection. Additionally, the data processing terminal 105 mayfurther be connected to a data network (not shown) for storing,retrieving, updating, and/or analyzing data corresponding to thedetected analyte level of the user.

The data processing terminal 105 may include a drug delivery device(e.g., an infusion device), such as an insulin infusion pump or thelike, which may be configured to administer a drug (e.g., insulin) tothe user, and which may be configured to communicate with the primaryreceiver unit 104 for receiving, among others, the measured analytelevel. Alternatively, the primary receiver unit 104 may be configured tointegrate an infusion device therein so that the primary receiver unit104 is configured to administer an appropriate drug (e.g., insulin) tousers, for example, for administering and modifying basal profiles, aswell as for determining appropriate boluses for administration based on,among others, the detected analyte levels received from the dataprocessing unit 102. An infusion device may be an external device or aninternal device, such as a device wholly implantable in a user.

In certain embodiments, the data processing terminal 105, which mayinclude an infusion device, e.g., an insulin pump, may be configured toreceive the analyte signals from the data processing unit 102, and thus,incorporate the functions of the primary receiver unit 104 includingdata processing for managing the user's insulin therapy and analytemonitoring. In certain embodiments, the communication link 103, as wellas one or more of the other communication interfaces shown in FIG. 1,may use one or more wireless communication protocols, such as, but notlimited to: an RF communication protocol, an infrared communicationprotocol, a Bluetooth enabled communication protocol, an 802.11xwireless communication protocol, or an equivalent wireless communicationprotocol which would allow secure, wireless communication of severalunits (for example, per Health Insurance Portability and AccountabilityAct (HIPPA) requirements), while avoiding potential data collision andinterference.

FIG. 2 shows a block diagram of an embodiment of a data processing unit102 of the analyte monitoring system shown in FIG. 1. User input and/orinterface components may be included or a data processing unit may befree of user input and/or interface components. In certain embodiments,one or more application-specific integrated circuits (ASIC) may be usedto implement one or more functions or routines associated with theoperations of the data processing unit (and/or receiver unit) using forexample one or more state machines and buffers.

As can be seen in the embodiment of FIG. 2, the analyte sensor 101(FIG. 1) includes four contacts, three of which are electrodes: a workelectrode (W) 210, a reference electrode (R) 212, and a counterelectrode (C) 213, each operatively coupled to the analog interface 201of the data processing unit 102. This embodiment also shows an optionalguard contact (G) 211. Fewer or greater electrodes may be employed. Forexample, the counter and reference electrode functions may be served bya single counter/reference electrode. In some cases, there may be morethan one working electrode and/or reference electrode and/or counterelectrode, etc.

FIG. 3 is a block diagram of an embodiment of a receiver/monitor unitsuch as the primary receiver unit 104 of the analyte monitoring systemshown in FIG. 1. The primary receiver unit 104 includes one or more of:a test strip interface 301, an RF receiver 302, a user input 303, anoptional temperature detection section 304, and a clock 305, each ofwhich is operatively coupled to a processing and storage section 307.The primary receiver unit 104 also includes a power supply 306operatively coupled to a power conversion and monitoring section 308.Further, the power conversion and monitoring section 308 is also coupledto the processing and storage section 307. Moreover, also shown are areceiver serial communication section 309, and an output 310, eachoperatively coupled to the processing and storage section 307. Theprimary receiver unit 104 may include user input and/or interfacecomponents or may be free of user input and/or interface components.

In certain embodiments, the test strip interface 301 includes an analytetesting portion (e.g., a glucose level testing portion) to receive ablood (or other body fluid sample) analyte test or information relatedthereto. For example, the test strip interface 301 may include a teststrip port to receive a test strip (e.g., a glucose test strip). Thedevice may determine the analyte level of the test strip, and optionallydisplay (or otherwise notice) the analyte level on the output 310 of theprimary receiver unit 104. Any suitable test strip may be employed,e.g., test strips that only require a very small amount (e.g., 3microliters or less, e.g., 1 microliter or less, e.g., 0.5 microlitersor less, e.g., 0.1 microliters or less), of applied sample to the stripin order to obtain accurate glucose information. Embodiments of teststrips include, e.g., Freestyle® blood glucose test strips from AbbottDiabetes Care, Inc. (Alameda, Calif.). Glucose information obtained byan in vitro glucose testing device may be used for a variety ofpurposes, computations, etc. For example, the information may be used tocalibrate sensor 101, confirm results of sensor 101 to increase theconfidence thereof (e.g., in instances in which information obtained bysensor 101 is employed in therapy related decisions), etc.

In further embodiments, the data processing unit 102 and/or the primaryreceiver unit 104 and/or the secondary receiver unit 106, and/or thedata processing terminal/infusion device 105 may be configured toreceive the analyte value wirelessly over a communication link from, forexample, a blood glucose meter. In further embodiments, a usermanipulating or using the analyte monitoring system 100 (FIG. 1) maymanually input the analyte value using, for example, a user interface(for example, a keyboard, keypad, voice commands, and the like)incorporated in one or more of the data processing unit 102, the primaryreceiver unit 104, secondary receiver unit 106, or the data processingterminal/infusion device 105.

Additional detailed descriptions are provided in U.S. Pat. Nos.5,262,035; 5,264,104; 5,262,305; 5,320,715; 5,593,852; 6,175,752;6,650,471; 6,746, 582, and 7,811,231, each of which is incorporatedherein by reference in their entirety.

FIG. 4 schematically shows an embodiment of an analyte sensor 400 inaccordance with the embodiments of the present disclosure. This sensorembodiment includes electrodes 401, 402 and 403 on a base 404.Electrodes (and/or other features) may be applied or otherwise processedusing any suitable technology, e.g., chemical vapor deposition (CVD),physical vapor deposition, sputtering, reactive sputtering, printing,coating, ablating (e.g., laser ablation), painting, dip coating,etching, and the like. Materials include, but are not limited to, anyone or more of aluminum, carbon (including graphite), cobalt, copper,gallium, gold, indium, iridium, iron, lead, magnesium, mercury (as anamalgam), nickel, niobium, osmium, palladium, platinum, rhenium,rhodium, selenium, silicon (e.g., doped polycrystalline silicon),silver, tantalum, tin, titanium, tungsten, uranium, vanadium, zinc,zirconium, mixtures thereof, and alloys, oxides, or metallic compoundsof these elements.

The analyte sensor 400 may be wholly implantable in a user or may beconfigured so that only a portion is positioned within (internal) a userand another portion outside (external) a user. For example, the sensor400 may include a first portion positionable above a surface of the skin410, and a second portion positioned below the surface of the skin. Insuch embodiments, the external portion may include contacts (connectedto respective electrodes of the second portion by traces) to connect toanother device also external to the user such as a transmitter unit.While the embodiment of FIG. 4 shows three electrodes side-by-side onthe same surface of base 404, other configurations are contemplated,e.g., fewer or greater electrodes, some or all electrodes on differentsurfaces of the base or present on another base, some or all electrodesstacked together, electrodes of differing materials and dimensions, etc.

FIG. 5A shows a perspective view of an embodiment of an analyte sensor500 having a first portion (which in this embodiment may becharacterized as a major portion) positionable above a surface of theskin 510, and a second portion (which in this embodiment may becharacterized as a minor portion) that includes an insertion tip 530positionable below the surface of the skin, e.g., penetrating throughthe skin and into, e.g., the subcutaneous space 520, in contact with theuser's biofluid, such as interstitial fluid. Contact portions of aworking electrode 511, a reference electrode 512, and a counterelectrode 513 are positioned on the first portion of the sensor 500situated above the skin surface 510. A working electrode 501, areference electrode 502, and a counter electrode 503 are shown at thesecond portion of the sensor 500 and particularly at the insertion tip530. Traces may be provided from the electrodes at the tip to thecontact, as shown in FIG. 5A. It is to be understood that greater orfewer electrodes may be provided on a sensor. For example, a sensor mayinclude more than one working electrode and/or the counter and referenceelectrodes may be a single counter/reference electrode, etc.

FIG. 5B shows a cross sectional view of a portion of the sensor 500 ofFIG. 5A. The electrodes 501, 502 and 503, of the sensor 500 as well asthe substrate and the dielectric layers are provided in a layeredconfiguration or construction. For example, as shown in FIG. 5B, in oneembodiment, the sensor 500 (such as the analyte sensor unit 101 of FIG.1), includes a substrate layer 504, and a first conducting layer 501such as carbon, gold, etc., disposed on at least a portion of thesubstrate layer 504, and which may provide the working electrode. Alsoshown disposed on at least a portion of the first conducting layer 501is a sensing element 508. As described herein, two or more sensingelements may be provided on a sensing surface of the working electrode,where the two or more sensing elements are disposed laterally to eachother. For example, FIG. 5C shows a schematic view of a portion ofworking electrode 501. Working electrode 501 includes a plurality ofindividual sensing elements 508. The sensing elements 508 arediscontiguous, such that the sensing elements 508 are arranged into anarray of individual sensing elements 508 on the working electrode 501.

A first insulation layer 505, such as a first dielectric layer incertain embodiments, is disposed or layered on at least a portion of thefirst conducting layer 501, and further, a second conducting layer 509may be disposed or stacked on top of at least a portion of the firstinsulation layer (or dielectric layer) 505. As shown in FIG. 5B, thesecond conducting layer 509 may provide the reference electrode 502, asdescribed herein having an extended lifetime, which includes a layer ofredox polymer as described herein.

A second insulation layer 506, such as a second dielectric layer incertain embodiments, may be disposed or layered on at least a portion ofthe second conducting layer 509. Further, a third conducting layer 503may be disposed on at least a portion of the second insulation layer 506and may provide the counter electrode 503. Finally, a third insulationlayer 507 may be disposed or layered on at least a portion of the thirdconducting layer 503. In this manner, the sensor 500 may be layered suchthat at least a portion of each of the conducting layers is separated bya respective insulation layer (for example, a dielectric layer). Theembodiments of FIGS. 5A and 5B show the layers having different lengths.In certain instances, some or all of the layers may have the same ordifferent lengths and/or widths.

In certain embodiments, some or all of the electrodes 501, 502, 503 maybe provided on the same side of the substrate 504 in the layeredconstruction as described above, or alternatively, may be provided in aco-planar manner such that two or more electrodes may be positioned onthe same plane (e.g., side-by side (e.g., parallel) or angled relativeto each other) on the substrate 504. For example, co-planar electrodesmay include a suitable spacing therebetween and/or include a dielectricmaterial or insulation material disposed between the conductinglayers/electrodes. Furthermore, in certain embodiments, one or more ofthe electrodes 501, 502, 503 may be disposed on opposing sides of thesubstrate 504. In such embodiments, contact pads may be one the same ordifferent sides of the substrate. For example, an electrode may be on afirst side and its respective contact may be on a second side, e.g., atrace connecting the electrode and the contact may traverse through thesubstrate.

As noted above, analyte sensors may include an analyte-responsive enzymeto provide a sensing element. Some analytes, such as oxygen, can bedirectly electrooxidized or electroreduced on a sensor, and morespecifically at least on a working electrode of a sensor. Otheranalytes, such as glucose and lactate, require the presence of at leastone electron transfer agent and/or at least one catalyst to facilitatethe electrooxidation or electroreduction of the analyte. Catalysts mayalso be used for those analytes, such as oxygen, that can be directlyelectrooxidized or electroreduced on the working electrode. For theseanalytes, each working electrode includes a sensing element (see forexample sensing element 508 of FIG. 5B) proximate to or on a surface ofa working electrode. In many embodiments, a sensing element is formednear or on only a small portion of at least a working electrode.

Each sensing element includes one or more components constructed tofacilitate the electrochemical oxidation or reduction of the analyte.The sensing element may include, for example, a catalyst to catalyze areaction of the analyte and produce a response at the working electrode,an electron transfer agent to transfer electrons between the analyte andthe working electrode (or other component), or both.

A variety of different sensing element configurations may be used. Incertain embodiments, the sensing elements are deposited on theconductive material of a working electrode. The sensing elements mayextend beyond the conductive material of the working electrode. In somecases, the sensing elements may also extend over other electrodes, e.g.,over the counter electrode and/or reference electrode (orcounter/reference is provided). In other embodiments, the sensingelements are contained on the working electrode, such that the sensingelements do not extend beyond the conductive material of the workingelectrode.

Sensing elements that are in direct contact with the working electrodemay contain an electron transfer agent to transfer electrons directly orindirectly between the analyte and the working electrode, and/or acatalyst to facilitate a reaction of the analyte. For example, aglucose, lactate, or oxygen electrode may be formed having sensingelements which contain a catalyst, including glucose oxidase, glucosedehydrogenase, lactate oxidase, or laccase, respectively, and anelectron transfer agent that facilitates the electrooxidation of theglucose, lactate, or oxygen, respectively.

In other embodiments the sensing elements are not deposited directly onthe working electrode. Instead, the sensing elements 508 may be spacedapart from the working electrode, and separated from the workingelectrode, e.g., by a separation layer. A separation layer may includeone or more membranes or films or a physical distance. In addition toseparating the working electrode from the sensing elements, theseparation layer may also act as a mass transport limiting layer and/oran interferent eliminating layer and/or a biocompatible layer.

In certain embodiments which include more than one working electrode,one or more of the working electrodes may not have corresponding sensingelements, or may have sensing elements that do not contain one or morecomponents (e.g., an electron transfer agent and/or catalyst) needed toelectrolyze the analyte. Thus, the signal at this working electrode maycorrespond to background signal which may be removed from the analytesignal obtained from one or more other working electrodes that areassociated with fully-functional sensing elements by, for example,subtracting the signal.

In certain embodiments, the sensing elements include one or moreelectron transfer agents. Electron transfer agents that may be employedare electroreducible and electrooxidizable ions or molecules havingredox potentials that are a few hundred millivolts above or below theredox potential of the standard calomel electrode (SCE). The electrontransfer agent may be organic, organometallic, or inorganic. Examples oforganic redox species are quinones and species that in their oxidizedstate have quinoid structures, such as Nile blue and indophenol.Examples of organometallic redox species are metallocenes includingferrocene. Examples of inorganic redox species are hexacyanoferrate(III), ruthenium hexamine, etc. Additional examples include thosedescribed in U.S. Pat. Nos. 6,736,957, 7,501,053 and 7,754,093, thedisclosures of each of which are incorporated herein by reference intheir entirety.

In certain embodiments, electron transfer agents have structures orcharges which prevent or substantially reduce the diffusional loss ofthe electron transfer agent during the period of time that the sample isbeing analyzed. For example, electron transfer agents include but arenot limited to a redox species, e.g., bound to a polymer which can inturn be disposed on or near the working electrode. The bond between theredox species and the polymer may be covalent, coordinative, or ionic.Although any organic, organometallic or inorganic redox species may bebound to a polymer and used as an electron transfer agent, in certainembodiments the redox species is a transition metal compound or complex,e.g., osmium, ruthenium, iron, and cobalt compounds or complexes. Itwill be recognized that many redox species described for use with apolymeric component may also be used, without a polymeric component.

Embodiments of polymeric electron transfer agents may contain a redoxspecies covalently bound in a polymeric composition. An example of thistype of mediator is poly(vinylferrocene). Another type of electrontransfer agent contains an ionically-bound redox species. This type ofmediator may include a charged polymer coupled to an oppositely chargedredox species. Examples of this type of mediator include a negativelycharged polymer coupled to a positively charged redox species such as anosmium or ruthenium polypyridyl cation. Another example of anionically-bound mediator is a positively charged polymer includingquaternized poly(4-vinyl pyridine) or poly(1-vinyl imidazole) coupled toa negatively charged redox species such as ferricyanide or ferrocyanide.In other embodiments, electron transfer agents include a redox speciescoordinatively bound to a polymer. For example, the mediator may beformed by coordination of an osmium or cobalt 2,2′-bipyridyl complex topoly(1-vinyl imidazole) or poly(4-vinyl pyridine).

Suitable electron transfer agents are osmium transition metal complexeswith one or more ligands, each ligand having a nitrogen-containingheterocycle such as 2,2′-bipyridine, 1,10-phenanthroline, 1-methyl,2-pyridyl biimidazole, or derivatives thereof. The electron transferagents may also have one or more ligands covalently bound in a polymer,each ligand having at least one nitrogen-containing heterocycle, such aspyridine, imidazole, or derivatives thereof. One example of an electrontransfer agent includes (a) a polymer or copolymer having pyridine orimidazole functional groups and (b) osmium cations complexed with twoligands, each ligand containing 2,2′-bipyridine, 1,10-phenanthroline, orderivatives thereof, the two ligands not necessarily being the same.Some derivatives of 2,2′-bipyridine for complexation with the osmiumcation include but are not limited to 4,4′-dimethyl-2,2′-bipyridine andmono-, di-, and polyalkoxy-2,2′-bipyridines, including4,4′-dimethoxy-2,2′-bipyridine. Derivatives of 1,10-phenanthroline forcomplexation with the osmium cation include but are not limited to4,7-dimethyl-1,10-phenanthroline and mono, di-, andpolyalkoxy-1,10-phenanthrolines, such as4,7-dimethoxy-1,10-phenanthroline. Polymers for complexation with theosmium cation include but are not limited to polymers and copolymers ofpoly(1-vinyl imidazole) (referred to as “PVI”) and poly(4-vinylpyridine) (referred to as “PVP”). Suitable copolymer substituents ofpoly(1-vinyl imidazole) include acrylonitrile, acrylamide, andsubstituted or quaternized N-vinyl imidazole, e.g., electron transferagents with osmium complexed to a polymer or copolymer of poly(1-vinylimidazole).

Embodiments may employ electron transfer agents having a redox potentialranging from about −200 mV to about +200 mV versus the standard calomelelectrode (SCE). The sensing elements may also include a catalyst whichis capable of catalyzing a reaction of the analyte. The catalyst mayalso, in some embodiments, act as an electron transfer agent. Oneexample of a suitable catalyst is an enzyme which catalyzes a reactionof the analyte. For example, a catalyst, including a glucose oxidase,glucose dehydrogenase (e.g., pyrroloquinoline quinone (PQQ), dependentglucose dehydrogenase, flavine adenine dinucleotide (FAD) dependentglucose dehydrogenase, or nicotinamide adenine dinucleotide (NAD)dependent glucose dehydrogenase), may be used when the analyte ofinterest is glucose. A lactate oxidase or lactate dehydrogenase may beused when the analyte of interest is lactate. Laccase may be used whenthe analyte of interest is oxygen or when oxygen is generated orconsumed in response to a reaction of the analyte.

In certain embodiments, a catalyst may be attached to a polymer, crosslinking the catalyst with another electron transfer agent, which, asdescribed above, may be polymeric. A second catalyst may also be used incertain embodiments. This second catalyst may be used to catalyze areaction of a product compound resulting from the catalyzed reaction ofthe analyte. The second catalyst may operate with an electron transferagent to electrolyze the product compound to generate a signal at theworking electrode. Alternatively, a second catalyst may be provided inan interferent-eliminating layer to catalyze reactions that removeinterferents.

In certain embodiments, the sensor works at a low oxidizing potential,e.g., a potential of about +40 mV vs. Ag/AgCl. This sensing elementsuse, for example, an osmium (Os)-based mediator constructed for lowpotential operation. Accordingly, in certain embodiments the sensingelements are redox active components that include: (1) osmium-basedmediator molecules that include (bidente) ligands, and (2) glucoseoxidase enzyme molecules. These two constituents are combined togetherin the sensing elements of the sensor.

A mass transport limiting layer (not shown), e.g., an analyte fluxmodulating layer, may be included with the sensor to act as adiffusion-limiting barrier to reduce the rate of mass transport of theanalyte, for example, glucose or lactate, into the region around theworking electrodes. The mass transport limiting layers are useful inlimiting the flux of an analyte to a working electrode in anelectrochemical sensor so that the sensor is linearly responsive over alarge range of analyte concentrations and is easily calibrated. Masstransport limiting layers may include polymers and may be biocompatible.A mass transport limiting layer may provide many functions, e.g.,biocompatibility and/or interferent-eliminating functions, etc.

In certain embodiments, a mass transport limiting layer is a membranecomposed of crosslinked polymers containing heterocyclic nitrogengroups, such as polymers of polyvinylpyridine and polyvinylimidazole.Embodiments also include membranes that are made of a polyurethane, orpolyether urethane, or chemically related material, or membranes thatare made of silicone, and the like.

A membrane may be formed by crosslinking I a polymer, modified with azwitterionic moiety, a non-pyridine copolymer component, and optionallyanother moiety that is either hydrophilic or hydrophobic, and/or hasother desirable properties, in an alcohol-buffer solution. The modifiedpolymer may be made from a precursor polymer containing heterocyclicnitrogen groups. For example, a precursor polymer may bepolyvinylpyridine or polyvinylimidazole. Optionally, hydrophilic orhydrophobic modifiers may be used to “fine-tune” the permeability of theresulting membrane to an analyte of interest. Optional hydrophilicmodifiers, such as poly(ethylene glycol), hydroxyl or polyhydroxylmodifiers, may be used to enhance the biocompatibility of the polymer orthe resulting membrane.

A membrane may be formed in situ by applying an alcohol-buffer solutionof a crosslinker and a modified polymer over the enzyme-containingsensing elements and allowing the solution to cure for about one to twodays or other appropriate time period. The crosslinker-polymer solutionmay be applied over the sensing elements by placing a droplet ordroplets of the membrane solution on the sensor, by dipping the sensorinto the membrane solution, by spraying the membrane solution on thesensor, and the like. Generally, the thickness of the membrane iscontrolled by the concentration of the membrane solution, by the numberof droplets of the membrane solution applied, by the number of times thesensor is dipped in the membrane solution, by the volume of membranesolution sprayed on the sensor, or by any combination of these factors.A membrane applied in this manner may have any combination of thefollowing functions: (1) mass transport limitation, i.e., reduction ofthe flux of analyte that can reach the sensing elements, (2)biocompatibility enhancement, or (3) interferent reduction.

In some instances, the membrane may form one or more bonds with thesensing elements. By bonds is meant any type of an interaction betweenatoms or molecules that allows chemical compounds to form associationswith each other, such as, but not limited to, covalent bonds, ionicbonds, dipole-dipole interactions, hydrogen bonds, London dispersionforces, and the like. For example, in situ polymerization of themembrane can form crosslinks between the polymers of the membrane andthe polymers in the sensing elements. In certain embodiments,crosslinking of the membrane to the sensing element facilitates areduction in the occurrence of delamination of the membrane from thesensor.

In certain embodiments, the sensing system detects hydrogen peroxide toinfer glucose levels. For example, a hydrogen peroxide-detecting sensormay be constructed in which the sensing elements include an enzyme suchas glucose oxidase, glucose dehydrogenase, or the like, and ispositioned on the working electrode. The sensing elements may be coveredby one or more layers, e.g., a membrane that is selectively permeable toglucose. Once the glucose passes through the membrane, it is oxidized bythe enzyme and reduced glucose oxidase can then be oxidized by reactingwith molecular oxygen to produce hydrogen peroxide.

Certain embodiments include a hydrogen peroxide-detecting sensorconstructed from sensing elements prepared by combining together, forexample: (1) a redox mediator having a transition metal complexincluding an Os polypyridyl complex with oxidation potentials of about+200 mV vs. SCE, and (2) periodate oxidized horseradish peroxidase(HRP). Such a sensor functions in a reductive mode; the workingelectrode is controlled at a potential negative to that of the Oscomplex, resulting in mediated reduction of hydrogen peroxide throughthe HRP catalyst.

In another example, a potentiometric sensor can be constructed asfollows. Glucose-sensing elements may be constructed by combiningtogether (1) a redox mediator having a transition metal complexincluding Os polypyridyl complexes with oxidation potentials from about−200 mV to +200 mV vs. SCE, and (2) glucose oxidase. This sensor canthen be used in a potentiometric mode, by exposing the sensor to aglucose containing solution, under conditions of zero current flow, andallowing the ratio of reduced/oxidized Os to reach an equilibrium value.The reduced/oxidized Os ratio varies in a reproducible way with theglucose concentration, and will cause the electrode's potential to varyin a similar way.

The substrate may be formed using a variety of non-conducting materials,including, for example, polymeric or plastic materials and ceramicmaterials. Suitable materials for a particular sensor may be determined,at least in part, based on the desired use of the sensor and propertiesof the materials.

In some embodiments, the substrate is flexible. For example, if thesensor is configured for implantation into a user, then the sensor maybe made flexible (although rigid sensors may also be used forimplantable sensors) to reduce pain to the user and damage to the tissuecaused by the implantation of and/or the wearing of the sensor. Aflexible substrate often increases the user's comfort and allows a widerrange of activities. Suitable materials for a flexible substrateinclude, for example, non-conducting plastic or polymeric materials andother non-conducting, flexible, deformable materials. Examples of usefulplastic or polymeric materials include thermoplastics such aspolycarbonates, polyesters (e.g., Mylar and polyethylene terephthalate(PET)), polyvinyl chloride (PVC), polyurethanes, polyethers, polyamides,polyimides, or copolymers of these thermoplastics, such as PETG(glycol-modified polyethylene terephthalate).

In other embodiments, the sensors are made using a relatively rigidsubstrate to, for example, provide structural support against bending orbreaking. Examples of rigid materials that may be used as the substrateinclude poorly conducting ceramics, such as aluminum oxide and silicondioxide. An implantable sensor having a rigid substrate may have a sharppoint and/or a sharp edge to aid in implantation of a sensor without anadditional insertion device.

It will be appreciated that for many sensors and sensor applications,both rigid and flexible sensors will operate adequately. The flexibilityof the sensor may also be controlled and varied along a continuum bychanging, for example, the composition and/or thickness of thesubstrate.

In addition to considerations regarding flexibility, it is oftendesirable that implantable sensors should have a substrate which isphysiologically harmless, for example, a substrate approved by aregulatory agency or private institution for in vivo use.

The sensor may include optional features to facilitate insertion of animplantable sensor. For example, the sensor may be pointed at the tip toease insertion. In addition, the sensor may include a barb which assistsin anchoring the sensor within the tissue of the user during operationof the sensor. However, the barb is typically small enough so thatlittle damage is caused to the subcutaneous tissue when the sensor isremoved for replacement.

An implantable sensor may also, optionally, have an anticlotting agentdisposed on a portion of the substrate which is implanted into a user.This anticlotting agent may reduce or eliminate the clotting of blood orother body fluid around the sensor, particularly after insertion of thesensor. Blood clots may foul the sensor or irreproducibly reduce theamount of analyte which diffuses into the sensor. Examples of usefulanticlotting agents include heparin and tissue plasminogen activator(TPA), as well as other known anticlotting agents.

The anticlotting agent may be applied to at least a portion of that partof the sensor that is to be implanted. The anticlotting agent may beapplied, for example, by bath, spraying, brushing, or dipping, etc. Theanticlotting agent is allowed to dry on the sensor. The anticlottingagent may be immobilized on the surface of the sensor or it may beallowed to diffuse away from the sensor surface. The quantities ofanticlotting agent disposed on the sensor may be below the amountstypically used for treatment of medical conditions involving blood clotsand, therefore, have only a limited, localized effect.

Insertion Device

An insertion device can be used to subcutaneously insert the sensor intothe user. The insertion device is typically formed using structurallyrigid materials, such as metal or rigid plastic. Materials may includestainless steel and ABS (acrylonitrile-butadiene-styrene) plastic. Insome embodiments, the insertion device is pointed and/or sharp at thetip to facilitate penetration of the skin of the user. A sharp, thininsertion device may reduce pain felt by the user upon insertion of thesensor. In other embodiments, the tip of the insertion device has othershapes, including a blunt or flat shape. These embodiments may be usefulwhen the insertion device does not penetrate the skin but rather servesas a structural support for the sensor as the sensor is pushed into theskin.

Sensor Control Unit

The sensor control unit can be integrated in the sensor, part or all ofwhich is subcutaneously implanted or it can be configured to be placedon the skin of a user. The sensor control unit is optionally formed in ashape that is comfortable to the user and which may permit concealment,for example, under a user's clothing. The thigh, leg, upper arm,shoulder, or abdomen are convenient parts of the user's body forplacement of the sensor control unit to maintain concealment. However,the sensor control unit may be positioned on other portions of theuser's body. One embodiment of the sensor control unit has a thin, ovalshape to enhance concealment. However, other shapes and sizes may beused.

The particular profile, as well as the height, width, length, weight,and volume of the sensor control unit may vary and depends, at least inpart, on the components and associated functions included in the sensorcontrol unit. In general, the sensor control unit includes a housingtypically formed as a single integral unit that rests on the skin of theuser. The housing typically contains most or all of the electroniccomponents of the sensor control unit.

The housing of the sensor control unit may be formed using a variety ofmaterials, including, for example, plastic and polymeric materials, suchas rigid thermoplastics and engineering thermoplastics. Suitablematerials include, for example, polyvinyl chloride, polyethylene,polypropylene, polystyrene, ABS polymers, and copolymers thereof. Thehousing of the sensor control unit may be formed using a variety oftechniques including, for example, injection molding, compressionmolding, casting, and other molding methods. Hollow or recessed regionsmay be formed in the housing of the sensor control unit. The electroniccomponents of the sensor control unit and/or other items, including abattery or a speaker for an audible alarm, may be placed in the hollowor recessed areas.

The sensor control unit is typically attached to the skin of the user,for example, by adhering the sensor control unit directly to the skin ofthe user with an adhesive provided on at least a portion of the housingof the sensor control unit which contacts the skin or by suturing thesensor control unit to the skin through suture openings in the sensorcontrol unit.

When positioned on the skin of a user, the sensor and the electroniccomponents within the sensor control unit are coupled via conductivecontacts. The one or more working electrodes, counter electrode (orcounter/reference electrode), optional reference electrode, and optionaltemperature probe are attached to individual conductive contacts. Forexample, the conductive contacts are provided on the interior of thesensor control unit. Other embodiments of the sensor control unit havethe conductive contacts disposed on the exterior of the housing. Theplacement of the conductive contacts is such that they are in contactwith the contact pads on the sensor when the sensor is properlypositioned within the sensor control unit.

Sensor Control Unit Electronics

The sensor control unit also typically includes at least a portion ofthe electronic components that operate the sensor and the analytemonitoring device system. The electronic components of the sensorcontrol unit typically include a power supply for operating the sensorcontrol unit and the sensor, a sensor circuit for obtaining signals fromand operating the sensor, a measurement circuit that converts sensorsignals to a desired format, and a processing circuit that, at minimum,obtains signals from the sensor circuit and/or measurement circuit andprovides the signals to an optional transmitter. In some embodiments,the processing circuit may also partially or completely evaluate thesignals from the sensor and convey the resulting data to the optionaltransmitter and/or activate an optional alarm system if the analytelevel exceeds a threshold. The processing circuit often includes digitallogic circuitry.

The sensor control unit may optionally contain a transmitter fortransmitting the sensor signals or processed data from the processingcircuit to a receiver/display unit; a data storage unit for temporarilyor permanently storing data from the processing circuit; a temperatureprobe circuit for receiving signals from and operating a temperatureprobe; a reference voltage generator for providing a reference voltagefor comparison with sensor-generated signals; and/or a watchdog circuitthat monitors the operation of the electronic components in the sensorcontrol unit.

Moreover, the sensor control unit may also include digital and/or analogcomponents utilizing semiconductor devices, including transistors. Tooperate these semiconductor devices, the sensor control unit may includeother components including, for example, a bias control generator tocorrectly bias analog and digital semiconductor devices, an oscillatorto provide a clock signal, and a digital logic and timing component toprovide timing signals and logic operations for the digital componentsof the circuit.

As an example of the operation of these components, the sensor circuitand the optional temperature probe circuit provide raw signals from thesensor to the measurement circuit. The measurement circuit converts theraw signals to a desired format, using for example, a current-to-voltageconverter, current-to-frequency converter, and/or a binary counter orother indicator that produces a signal proportional to the absolutevalue of the raw signal. This may be used, for example, to convert theraw signal to a format that can be used by digital logic circuits. Theprocessing circuit may then, optionally, evaluate the data and providecommands to operate the electronics.

Calibration

Sensors may be configured to require no system calibration or no usercalibration. For example, a sensor may be factory calibrated and neednot require further calibrating. In certain embodiments, calibration maybe required, but may be done without user intervention, i.e., may beautomatic. In those embodiments in which calibration by the user isrequired, the calibration may be according to a predetermined scheduleor may be dynamic, i.e., the time for which may be determined by thesystem on a real-time basis according to various factors, including, butnot limited to, glucose concentration and/or temperature and/or rate ofchange of glucose, etc.

In addition to a transmitter, an optional receiver may be included inthe sensor control unit. In some cases, the transmitter is atransceiver, operating as both a transmitter and a receiver. Thereceiver may be used to receive calibration data for the sensor. Thecalibration data may be used by the processing circuit to correctsignals from the sensor. This calibration data may be transmitted by thereceiver/display unit or from some other source such as a control unitin a doctor's office. In addition, the optional receiver may be used toreceive a signal from the receiver/display units to direct thetransmitter, for example, to change frequencies or frequency bands, toactivate or deactivate the optional alarm system and/or to direct thetransmitter to transmit at a higher rate.

Calibration data may be obtained in a variety of ways. For instance, thecalibration data may be factory-determined calibration measurementswhich can be input into the sensor control unit using the receiver ormay alternatively be stored in a calibration data storage unit withinthe sensor control unit itself (in which case a receiver may not beneeded). The calibration data storage unit may be, for example, areadable or readable/writeable memory circuit. In some cases, a systemmay only need to be calibrated once during the manufacturing process,where recalibration of the system is not required.

If necessary, calibration may be accomplished using an in vitro teststrip (or other reference), e.g., a small sample test strip such as atest strip that requires less than about 1 microliter of sample (forexample FreeStyle® blood glucose monitoring test strips from AbbottDiabetes Care, Alameda, Calif.). For example, test strips that requireless than about 1 nanoliter of sample may be used. In certainembodiments, a sensor may be calibrated using only one sample of bodyfluid per calibration event. For example, a user need only lance a bodypart one time to obtain a sample for a calibration event (e.g., for atest strip), or may lance more than one time within a short period oftime if an insufficient volume of sample is firstly obtained.Embodiments include obtaining and using multiple samples of body fluidfor a given calibration event, where glucose values of each sample aresubstantially similar. Data obtained from a given calibration event maybe used independently to calibrate or combined with data obtained fromprevious calibration events, e.g., averaged including weighted averaged,etc., to calibrate. In certain embodiments, a system need only becalibrated once by a user, where recalibration of the system is notrequired.

Alternative or additional calibration data may be provided based ontests performed by a health care professional or by the user. Forexample, it is common for diabetic individuals to determine their ownblood glucose concentration using commercially available testing kits.The results of this test is input into the sensor control unit eitherdirectly, if an appropriate input device (e.g., a keypad, an opticalsignal receiver, or a port for connection to a keypad or computer) isincorporated in the sensor control unit, or indirectly by inputting thecalibration data into the receiver/display unit and transmitting thecalibration data to the sensor control unit.

Other methods of independently determining analyte levels may also beused to obtain calibration data. This type of calibration data maysupplant or supplement factory-determined calibration values.

In some embodiments of the invention, calibration data may be requiredat periodic intervals, for example, every eight hours, once a day, oronce a week, to confirm that accurate analyte levels are being reported.Calibration may also be required each time a new sensor is implanted orif the sensor exceeds a threshold minimum or maximum value or if therate of change in the sensor signal exceeds a threshold value. In somecases, it may be necessary to wait a period of time after theimplantation of the sensor before calibrating to allow the sensor toachieve equilibrium. In some embodiments, the sensor is calibrated onlyafter it has been inserted. In other embodiments, no calibration of thesensor is needed.

Analyte Monitoring Device

In some embodiments of the invention, the analyte monitoring deviceincludes a sensor control unit and a sensor. In these embodiments, theprocessing circuit of the sensor control unit is able to determine alevel of the analyte and activate an alarm system if the analyte levelexceeds a threshold value. The sensor control unit, in theseembodiments, has an alarm system and may also include a display, such asan LCD or LED display.

A threshold value is exceeded if the datapoint has a value that isbeyond the threshold value in a direction indicating a particularcondition. For example, a datapoint which correlates to a glucose levelof 200 mg/dl exceeds a threshold value for hyperglycemia of 180 mg/dL,because the datapoint indicates that the user has entered ahyperglycemic state. As another example, a datapoint which correlates toa glucose level of 65 mg/dl exceeds a threshold value for hypoglycemiaof 70 mg/dL because the datapoint indicates that the user ishypoglycemic as defined by the threshold value. However, a datapointwhich correlates to a glucose level of 75 mg/dl would not exceed thesame threshold value of 70 mg/dL for hypoglycemia because the datapointdoes not indicate that particular condition as defined by the chosenthreshold value.

An alarm may also be activated if the sensor readings indicate a valuethat is outside of (e.g., above or below) a measurement range of thesensor. For glucose, the physiologically relevant measurement range istypically 30-400 mg/dL, including 40-300 mg/dl and 50-250 mg/dl, ofglucose in the interstitial fluid.

The alarm system may also, or alternatively, be activated when the rateof change or acceleration of the rate of change in analyte levelincrease or decrease reaches or exceeds a threshold rate oracceleration. For example, in the case of a subcutaneous glucosemonitor, the alarm system may be activated if the rate of change inglucose concentration exceeds a threshold value which may indicate thata hyperglycemic or hypoglycemic condition is likely to occur. In somecases, the alarm system is activated if the acceleration of the rate ofchange in glucose concentration exceeds a threshold value which mayindicate that a hyperglycemic or hypoglycemic condition is likely tooccur.

A system may also include system alarms that notify a user of systeminformation such as battery condition, calibration, sensor dislodgment,sensor malfunction, etc. Alarms may be, for example, auditory and/orvisual. Other sensory-stimulating alarm systems may be used includingalarm systems which heat, cool, vibrate, or produce a mild electricalshock when activated.

Drug Delivery System

The subject invention also includes sensors used in sensor-based drugdelivery systems. The system may provide a drug to counteract the highor low level of the analyte in response to the signals from one or moresensors. Alternatively, the system may monitor the drug concentration toensure that the drug remains within a desired therapeutic range. Thedrug delivery system may include one or more (e.g., two or more)sensors, a processing unit such as a transmitter, a receiver/displayunit, and a drug administration system. In some cases, some or allcomponents may be integrated in a single unit. A sensor-based drugdelivery system may use data from the one or more sensors to providenecessary input for a control algorithm/mechanism to adjust theadministration of drugs, e.g., automatically or semi-automatically. Asan example, a glucose sensor may be used to control and adjust theadministration of insulin from an external or implanted insulin pump.

Each of the various references, presentations, publications, provisionaland/or non-provisional U.S. Patent Applications, U.S. Patents, non-U.S.Patent Applications, and/or non-U.S. Patents that have been identifiedherein, is incorporated herein by reference in its entirety.

Other embodiments and modifications within the scope of the presentdisclosure will be apparent to those skilled in the relevant art.Various modifications, processes, as well as numerous structures towhich the embodiments of the invention may be applicable will be readilyapparent to those of skill in the art to which the invention is directedupon review of the specification. Various aspects and features of theinvention may have been explained or described in relation tounderstandings, beliefs, theories, underlying assumptions, and/orworking or prophetic examples, although it will be understood that theinvention is not bound to any particular understanding, belief, theory,underlying assumption, and/or working or prophetic example. Althoughvarious aspects and features of the invention may have been describedlargely with respect to applications, or more specifically, medicalapplications, involving diabetic humans, it will be understood that suchaspects and features also relate to any of a variety of applicationsinvolving non-diabetic humans and any and all other animals. Further,although various aspects and features of the invention may have beendescribed largely with respect to applications involving partiallyimplanted sensors, such as transcutaneous or subcutaneous sensors, itwill be understood that such aspects and features also relate to any ofa variety of sensors that are suitable for use in connection with thebody of an animal or a human, such as those suitable for use as fullyimplanted in the body of an animal or a human. Finally, although thevarious aspects and features of the invention have been described withrespect to various embodiments and specific examples herein, all ofwhich may be made or carried out conventionally, it will be understoodthat the invention is entitled to protection within the full scope ofthe appended claims.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how tomake and use the embodiments of the invention, and are not intended tolimit the scope of what the inventors regard as their invention nor arethey intended to represent that the experiments below are all or theonly experiments performed. Efforts have been made to ensure accuracywith respect to numbers used (e.g., amounts, temperature, etc.) but someexperimental errors and deviations should be accounted for. Unlessindicated otherwise, parts are parts by weight, molecular weight isweight average molecular weight, temperature is in degrees Centigrade,and pressure is at or near atmospheric.

EXAMPLES Example 1

FIG. 6 shows a photograph of a working electrode coated with six sensingelements (labeled 1 to 6) with a radius of approximately 150 μm each ata distance of approximately 150 μm from each other. The resultingsensors have a coefficient of variation in sensitivity of 5% or less.The diameters of each sensing element in FIG. 6 are shown in Table 1below.

TABLE 1 Sensing Element Diameter (μm) 1 146.85 2 156.50 3 153.91 4165.58 5 145.04 6 166.89

Example 2

FIG. 10 shows a graph of current (μA) vs. time (seconds) for a sensinglayer formulation deposited as an array of small sensing elements (9pL/droplet), and a sensing layer formulation deposited as a stripecoating on the surface of a gold working electrode. The sensor with thesensing layer formulation deposited as an array of small sensingelements recovered 26% more charge than the stripe-coated sensor underthe same test conditions.

1. What is claimed is:
 1. A method of manufacturing an analyte sensorcomprising: providing a substrate; and depositing an array ofdiscontiguous sensing elements to a surface of the substrate of theanalyte sensor, the sensing elements comprising one or more droplets ofa sensing element formulation.
 2. The method of claim 1, wherein thesurface of the substrate is a surface of a working electrode.
 3. Themethod of claim 1, wherein the depositing is performed by a non-impactprinting method.
 4. The method of claim 3, wherein the non-impactprinting method comprises a piezoelectric pulse-jet deposition.
 5. Themethod of claim 3, wherein the non-impact printing method comprises athermoelectric pulse-jet deposition.
 6. The method of claim 1, whereinthe depositing is performed by an impact printing method.
 7. The methodof claim 2, wherein a droplet deposited during a single activation eventof the non-impact printing method has a volume ranging from 0.01 pL to1000 pL.
 8. The method of claim 1, wherein the depositing comprisesdepositing a first layer of the sensing elements to the surface of thesubstrate and depositing a second layer of the sensing elements, eachsensing element of the second layer being substantially aligned with aninter-feature area between the sensing elements of the first layer. 9.The method of claim 1, wherein the array contains a plurality of rows ofdiscontiguous sensing elements, each row being substantially alignedwith adjacent rows.
 10. The method of claim 1, wherein the arraycontains a plurality of rows of discontiguous sensing elements, each rowbeing offset from adjacent rows.
 11. The method of claim 1, wherein thesensing element formulation comprises an analyte-responsive enzyme. 12.The method of claim 11, wherein the analyte-responsive enzyme comprisesa glucose-responsive enzyme.
 13. The method of claim 12, wherein theglucose-responsive enzyme comprises one or more of a glucose oxidase, aglucose dehydrogenase, a pyrroloquinoline quinone, a dependent glucosedehydrogenase, a flavine adenine dinucleotide dependent glucosedehydrogenase, or a nicotinamide adenine dinucleotide dependent glucosedehydrogenase.
 14. The method of claim 12, wherein theanalyte-responsive enzyme comprises a lactate-responsive enzyme.
 15. Themethod of claim 12, wherein the analyte-responsive enzyme comprises anoxygen-responsive enzyme.
 16. The method of claim 1, wherein the analytesensor is configured to monitor glucose.
 17. The method of claim 1,wherein the analyte sensor is configured to monitor ketones.
 18. Themethod of claim 1, wherein the analyte sensor is configured to monitorone or more of acetyl choline, amylase, bilirubin, cholesterol,chorionic gonadotropin, glycosylated hemoglobin (HbA1c), creatine kinase(e.g., CK-MB), creatine, creatinine, DNA, fructosamine, glucose, glucosederivatives, glutamine, growth hormones, hormones, ketones, ketonebodies, lactate, peroxide, prostate-specific antigen, prothrombin, RNA,thyroid stimulating hormone, and troponin. The concentration of drugs,such as, for example, antibiotics (e.g., gentamicin, vancomycin, and thelike), digitoxin, digoxin, drugs of abuse, theophylline, and warfarin.19. The method of claim 1, wherein the sensing elements have an averagediameter ranging from 0.1 μm to 500 μm.
 20. The method of claim 1,wherein the sensing elements have a volume ranging from 0.01 pL to 1000pL.
 21. The method of claim 1, wherein the array comprises a number ofdiscontiguous sensing elements in a range of 2 to 1000 discontiguoussensing elements.
 22. The method of claim 1, further comprising forminga membrane over the array of discontiguous sensing elements by dipping aportion of the analyte sensor into a membrane solution, wherein themembrane limits flux of analyte to the sensing elements.
 23. The methodof claim 1, further comprising forming a membrane over the array ofdiscontiguous sensing elements by spraying a portion of the analytesensor with a membrane solution, wherein the membrane limits flux ofanalyte to the sensing elements.
 24. The method of claim 1, wherein thearray of discontiguous sensing elements comprises an inter-featuredistance between each of the discontiguous sensing elements ranging from1 μm to 500 μm.